In late years, a transparent organic polymer material has been used as a transparent material in place of an inorganic glass, and has been used, for example, for an optical resin or the like. There has been demanded an optical resin having required general properties such as transparency, thermal properties, mechanical properties and the like, while attaining a high refractive index.
A conventional technique concerning such a resin has been disclosed in Patent Document 1 (International Publication Pamphlet No. 2005/095490). In the Document, a thietane compound containing a metal has been disclosed. Furthermore, an optical resin having a high refractive index exceeding a refractive index (nd) of 1.7 has been disclosed.    Patent Document 1: International Publication Pamphlet No. 2005/095490    Patent Document 2: Japanese Patent Laid-open No. 2003-327583